_PATTERN-MAKING AND CASTING._
Patterns and castings are so intimately connected that it would
be difficult to treat of them separately without continually
confounding them together; it is therefore proposed to speak of
pattern-making and moulding under one head.
Every operation in a pattern-shop has reference to some operation
in the foundry, and patterns considered separately from moulding
operations would be incomprehensible to any but the skilled.
Next to designing and draughting, pattern-making is the most
intellectual of what may be termed engineering processes--the
department that must include an exercise of the greatest amount of
personal judgment on the part of the workman, and at the same time
demands a high grade of hand skill.
For other kinds of work there are drawings furnished, and the plans
are dictated by the engineering department of machinery-building
establishments, but pattern-makers make their own plans for
constructing their work, and have even to reproduce the drawings
of the fitting-shop to work from. Nearly everything pertaining to
patterns is left to be decided by the pattern-maker, who, from the
same drawings, and through the exercise of his judgment alone, may
make patterns that are durable and expensive, or temporary and
cheap, as the probable extent of their use may determine.
The expense of patterns should be divided among and charged to the
machines for which the patterns are employed, but there can be
no constant rules for assessing or dividing this cost. A pattern
may be employed but once, or it may be used for years; it is
continually liable to be superseded by changes and improvements
that cannot be predicted beforehand; and in preparing patterns,
the question continually arises of how much ought to be expended
on them--a matter that should be determined between the engineer
and the pattern-maker, but is generally left to the pattern-maker
alone, for the reason that but few mechanical engineers understand
pattern-making so well as to dictate plans of construction.
To point out some of the leading points or conditions to be taken
into account in pattern-making, and which must be understood
in order to manage this department, I will refer to them in
consecutive order.
_First._--Durability, plans of construction and cost, which all
amount to the same thing. To determine this point, there is to
be considered the amount of use that the patterns are likely to
serve, whether they are for standard or special machines, and the
quality of the castings so far as affected by the patterns. A
first-class pattern, framed to withstand moisture and rapping, may
cost twice as much as another that has the same outline, yet the
cheaper pattern may answer almost as well to form a few moulds as
an expensive one.
_Second._--The manner of moulding and its expense, so far as
determined by the patterns, which may be parted so as to be 'rammed
up' on fallow boards or a level floor, or the patterns may be
solid, and have to be bedded, as it is termed; pieces on the top
may be made loose, or fastened on so as to 'cope off;' patterns may
be well finished so as to draw clean, or rough so that a mould may
require a great deal of time to dress up after a pattern is removed.
_Third._--The soundness of such parts as are to be planed, bored,
and turned in finishing; this is also a matter that is determined
mainly by how the patterns are arranged, by which is the top
and which the bottom or drag side, the manner of drawing, and
provisions for avoiding dirt and slag.
_Fourth._--Cores, where used, how vented, how supported in the
mould, and I will add how made, because cores that are of an
irregular form are often more expensive than external moulds,
including the patterns. The expense of patterns is often greatly
reduced, but is sometimes increased, by the use of cores, which may
be employed to cheapen patterns, add to their durability, or to
ensure sound castings.
_Fifth._--Shrinkage; the allowance that has to be made for the
contraction of castings in cooling, in other words, the difference
between the size of a pattern and the size of the casting. This is
a simple matter apparently, which may be provided for in allowing
a certain amount of shrinkage in all directions, but when the
inequalities of shrinkage both as to time and degree are taken into
account, the allowance to be made becomes a problem of no little
complication.
_Sixth._--Inherent, or cooling strains, that may either spring and
warp castings, or weaken them by maintained tension in certain
parts--a condition that often requires a disposition of the metal
quite different from what working strains demand.
_Seventh._--Draught, the bevel or inclination on the sides of
patterns to allow them to be withdrawn from the moulds without
dragging or breaking the sand.
_Eighth._--Rapping plates, draw plates, and lifting irons for
drawing the patterns out of the moulds; fallow and match boards,
with other details that are peculiar to patterns, and have no
counterparts, neither in names nor uses, outside the foundry.
This makes a statement in brief of what comprehends a knowledge
of pattern-making, and what must be understood not only by
pattern-makers, but also by mechanical engineers who undertake to
design machinery or manage its construction successfully.
As to the manner of cutting out or planing up the lumber for
patterns, and the manner of framing them together, it is useless to
devote space to the subject here; one hour's practical observation
in a pattern-shop, and another hour spent in examining different
kinds of patterns, is worth more to the apprentice than a whole
volume written to explain how these last-named operations are
performed. A pattern, unless finished with paint or opaque varnish,
will show the manner in which the wood is disposed in framing the
parts together.
I will now proceed to review these conditions or principles in
pattern-making and casting in a more detailed way, furnishing
as far as possible reasons for different modes of constructing
patterns, and the various plans of moulding and casting.
In regard to the character or quality of wood patterns, they can
be made, as already stated, at greater or less expense, and if
necessary, capable of almost any degree of endurance. The writer
has examined patterns which had been used more than two hundred
times, and were apparently good for an equal amount of use. Such
patterns are expensive in their first cost, but are the cheapest in
the end, if they are to be employed for a large number of castings.
Patterns for special pieces, or such as are to be used for a few
times only, do not require to be strong nor expensive, yet with
patterns, as with everything else pertaining to machinery, the
safest plan is to err on the side of strength.
For pulleys, gear wheels, or other standard parts of machinery
which are not likely to be modified or changed, iron patterns are
preferable; patterns for gear wheels and pulleys, when made of
wood, aside from their liability to spring and warp, cannot be made
sufficiently strong to withstand foundry use; besides, the greatest
accuracy that can be attained, even by metal patterns, is far from
producing true castings, especially for tooth wheels. The more
perfect patterns are, the less rapping is required in drawing them;
and the less rapping done, the more perfect castings will be.
The most perfect castings for gear wheels and pulleys and other
pieces which can be so moulded, are made by drawing the patterns
through templates without rapping. These templates are simply
plates of metal perforated so that the pattern can be forced
through them by screws or levers, leaving the sand intact. Such
templates are expensive to begin with, because of the accurate
fitting that is required, especially around the teeth of wheels,
and the mechanism that is required in drawing the patterns, but
when a large number of pieces are to be made from one pattern, such
as gear wheels and pulleys, the saving of labour will soon pay for
the templates and machinery required, to say nothing of the saving
of metal, which often amounts to ten per cent., and the increased
value of the castings because of their accuracy.
Mr Ransome of Ipswich, England, where this system of template
moulding originated, has invented a process of fitting templates
for gear wheels and other kinds of casting by pouring melted white
metal around to mould the fit instead of cutting it through the
templates; this effects a great saving in expense, and answers in
many cases quite as well as the old plan.
The expense of forming pattern-moulds may be considered as divided
between the foundry and pattern-shop. What a pattern-maker saves
a moulder may lose, and what a pattern-maker spends a moulder may
save; in other words, there is a point beyond which saving expense
in patterns is balanced by extra labour and waste in moulding--a
fact that is not generally realised because of inaccurate records
of both pattern and foundry work. What is lost or saved by
judicious or careless management in the matter of patterns and
moulding can only be known to those who are well skilled in both
moulding and pattern-making. A moulder may cut all the fillets in
a mould with a trowel; he may stop off, fill up, and print in, to
save pattern-work, but it is only expedient to do so when it costs
very much less than to prepare proper patterns, because patching
and cutting in moulds seldom improves them.
The reader may notice how everything pertaining to patterns and
moulding resolves itself into a matter of judgment on the part of
workmen, and how difficult it would be to apply general rules.
The arrangement of patterns with reference to having certain parts
of castings solid and clean is an important matter, yet one that is
comparatively easy to understand. Supposing the iron in a mould to
be in a melted state, and to contain, as it always must, loose sand
and 'scruff,' and that the weight of the dirt is to melted iron as
the weight of cork is to water, it is easy to see where this dirt
would lodge, and where it would be found in the castings. The top
of a mould or cope, as it is called, contains the dirt, while the
bottom or drag side is generally clean and sound: the rule is to
arrange patterns so that the surfaces to be finished will come on
the bottom or drag side.
Expedients to avoid dirt in such castings as are to be finished all
over or on two sides are various. Careful moulding to avoid loose
sand and washing is the first requisite; sinking heads, that rise
above the moulds, are commonly employed when castings are of a form
which allows the dirt to collect at one point. Moulds for sinking
heads are formed by moulders as a rule, but are sometimes provided
for by the patterns.
The quality of castings is governed by a great many things besides
what have been named, such as the manner of gating or flowing the
metal into the moulds, the temperature and quality of the iron, the
temperature and character of the mould--things which any skilled
foundryman will take pleasure in explaining in answer to courteous
and proper questions.
Cores are employed mainly for what may be termed the displacement
of metal in moulds. There is no clear line of distinction between
cores and moulds, as founding is now conducted; cores may be
of green sand, and made to surround the exterior of a piece,
as well as to make perforations or to form recesses within it.
The term 'core,' in its technical sense, means dried moulds, as
distinguished from green sand. Wheels or other castings are said
to be cast in cores when the moulds are made in pieces and dried.
Supporting and venting cores, and their expansion, are conditions
to which especial attention is called. When a core is surrounded
with hot metal, it gives off, because of moisture and the burning
of the 'wash,' a large amount of gas which must have free means
of escape. In the arrangement of cores, therefore, attention must
be had to some means of venting, which is generally attained
by allowing them to project through the sides of the mould and
communicate with the air outside.
An apprentice may get a clear idea of this venting process by
inspecting tubular core barrels, such as are employed in moulding
pipes or hollow columns, or by examining ordinary cores about a
foundry. Provision of some kind to 'carry off the vent,' as it is
termed by moulders, will be found in every case. The venting of
moulds is even more important than venting cores, because core
vents only carry off gas generated within the core itself, while
the gas from its exterior surface, and from the whole mould, has to
find means of escaping rapidly from the flasks when the hot metal
enters.
A learner will no doubt wonder why sand is used for moulding,
instead of some more adhesive material like clay. If he is not
too fastidious for the experiment, and will apply a lump of damp
moulding sand to his mouth and blow his breath through the mass,
the query will be solved. If it were not for the porous nature of
sand-moulds they would be blown to pieces as soon as the hot metal
entered them; not only because of the mechanical expansion of the
gas, but often from explosion by combustion. Gas jets from moulds,
as may be seen at any time when castings are poured, will take fire
and burn the same as illuminating gas.
If it were not for securing vent for gas, moulds could be made from
plastic material so as to produce fine castings with clear sharp
outlines.
The means of supporting cores must be devised, or at least
understood, by pattern-makers; these supports consist of 'prints'
and 'anchors.' Prints are extensions of the cores, which project
through the casting and extend into the sides of the mould, to
be held by the sand or by the flask. The prints of cores have
duplicates on the patterns, called core prints, which are, or
should be, of a different colour from the patterns, so as to
distinguish one from the other. The amount of surface required to
support cores is dependent upon their weight, or rather upon their
cubic contents, because the weight of a core is but a trifling
matter compared to its floating force when surrounded by melted
metal. An apprentice in studying devices for supporting cores must
remember that the main force required is to hold them down, and
not to bear their weight. The floating force of a core is as the
difference between its weight and that of a solid of metal of the
same size--a matter moulders often forget to consider. It is often
impossible, from the nature of castings, to have prints large
enough to support the cores, and it is then effected by anchors,
pieces of iron that stand like braces between the cores and the
flasks or pieces of iron imbedded in the sand to receive the strain
of the anchors.
In constructing patterns where it is optional whether to employ
cores or not, and in preparing drawings for castings which may
have either a ribbed or a cored section, it is nearly always best
to employ cores. The usual estimate of the difference between the
cost of moulding rib and cored sections, as well as of skeleton and
cored patterns, is wrong. The expense of cores is often balanced
by the advantage of having an 'open mould,' that is accessible for
repairs or facing, and by the greater durability and convenience of
the solid patterns. Taking, for example, a column, or box frame for
machinery, that might be made either with a rib or a cored section,
it would at first thought seem that patterns for a cored casting
would cost much more by reason of the core-boxes; but it must be
remembered that in most patterns labour is the principal expense,
and what is lost in the extra lumber required for a core-box or in
making a solid pattern is in many cases more than represented in
the greater amount of labour required to construct a rib pattern.
Cores expand when heated, and require an allowance in their
dimensions the reverse from patterns; this is especially the
case when the cores are made upon iron frames. For cylindrical
cores less than six inches diameter, or less than two feet long,
expansion need not be taken into account by pattern-makers, but for
large cores careful calculation is required. The expansion of cores
is as the amount of heat imparted to them, and the amount of heat
taken up is dependent upon the quantity of metal that may surround
the core and its conducting power.
Shrinkage, or the contraction of castings in cooling, is provided
for by adding from one-tenth to one-eighth of an inch to each
foot in the dimensions of patterns. This is a simple matter,
and is accomplished by employing a shrink rule in laying down
pattern-drawings from the figured dimensions of the finished work;
such rules are about one-hundredth part longer than the standard
scale.
This matter of shrinkage is indeed the only condition in
pattern-making which is governed by anything near a constant rule,
and even shrinkage requires sometimes to be varied to suit special
cases. For small patterns whose dimensions do not exceed one foot
in any direction, rapping will generally make up for shrinkage, and
no allowance is required in the patterns, but pattern-makers are so
partial to the rule of shrinkage, as the only constant one in their
work, that they are averse to admitting exceptions, and usually
keep to the shrink rule for all pieces, whether large or small.
Inherent or cooling strains in castings is much more intricate than
shrinkage: it is, in fact, one of the most uncertain and obscure
matters that pattern-makers and moulders have to contend with.
Inherent strains may weaken castings, or cause them to break while
cooling, or sometimes even after they are finished; and in many
kinds of works such strains must be carefully guarded against, both
in the preparation of designs and the arrangement of patterns,
especially for wheels and pulleys with spokes, and for struts or
braces with both ends fixed. The main difficulty resulting from
cooling strains, however, is that of castings being warped and
sprung; this difficulty is continually present in the foundry and
machine-shop, and there is perhaps no problem in the whole range
of mechanical manipulation of which there exists more diversity
of opinion and practice than of means to prevent the springing of
castings. This being the case, an apprentice can hardly hope for
much information here. There is no doubt of springing and strains
in castings being the result of constant causes that might be fully
understood if it were not for the ever-changing conditions which
exist in casting, both as to the form of pieces, the temperature
and quality of metal, mode of cooling, and so on.
Castings are of course sprung by the action of unequal strains,
caused by one part cooling or 'setting' sooner than another. That
far all is clear, but the next step takes us into the dark. What
are the various conditions which induce irregular cooling, and how
is it to be avoided?
Irregularity of cooling may be the result of unequal conducting
power in different parts of a mould or cores, or it may be from
the varying dimensions of the castings, which contain heat as
their thickness, and give it off in the same ratio. As a rule,
the drag or bottom side of a casting cools first, especially if a
mould rests on the ground, and there is not much sand between the
castings and the earth; this is a common cause of unequal cooling,
especially in large flat pieces. Air being a bad conductor of heat,
and the sand usually thin on the cope or top side, the result is
that the top of moulds remain quite hot, while at the bottom the
earth, being a good conductor, carries off the heat and cools that
side first, so that the iron 'sets' first on the bottom, afterwards
cooling and contracting on the top, so that castings are warped and
left with inherent strains.
These are but a few of many influences which tend to irregular
cooling, and are described with a view of giving a clue from
which other causes may be traced out. The want of uniformity in
sections which tends to irregular cooling can often be avoided
without much loss by a disposition of the metal with reference
to cooling strains. This, so far as the extra metal required to
give uniformity to or to balance the different sides of a casting,
is a waste which engineers are sometimes loth to consent to,
and often neglect in designs for moulded parts; yet, as before
said, the difficulty of irregular cooling can in a great degree
be counteracted by a proper distribution of the metal, without
wasting, if the matter is properly understood. No one is prepared
to make designs for castings who has not studied the subject of
cooling strains as thoroughly as possible, from practical examples
as well as by theoretical deductions.
Draught, or the taper required to allow patterns to be
drawn readily, is another of those indefinite conditions in
pattern-making that must be constantly decided by judgment and
experience. It is not uncommon to find rules for the draught of
patterns laid down in books, but it would be difficult to find
such rules applied. The draught may be one-sixteenth of an inch to
each foot of depth, or it may be one inch to a foot of depth, or
there may be no draught whatever. Any rule, considered aside from
specified conditions, will only confuse a learner. The only plan to
understand the proper amount of draught for patterns is to study
the matter in connection with patterns and foundry operations.
Patterns that are deep, and for castings that require to be
parallel or square when finished, are made with the least possible
amount of draught. If a pattern is a plain form, that affords
facilities for lifting or drawing, it may be drawn without taper
if its sides are smooth and well finished. Pieces that are shallow
and moulded often should, as a matter of convenience, have as much
taper as possible; and as the quantity of draught can be as the
depth of a pattern, we frequently see them made with a taper that
exceeds one inch to the foot of depth.
Moulders generally rap patterns as much as they will stand, often
more than they will stand; and in providing for draught it is
necessary to take these customs into account. There is no use
in making provision to save rapping unless the rapping is to be
omitted.
Rapping plates, draw-irons, and other details of pattern-making are
soon understood by observation. Perhaps the most useful suggestion
which can be given in reference to draw-irons is to say they should
be set on the under or bottom side of patterns, instead of on the
top, where they are generally placed. A draw-plate set in this way,
with a hole bored through the pattern so as to insert draw-irons
from the top, cannot pull off, which it is apt to do if set on the
top side. Every pattern no matter how small, should be ironed,
unless it is some trifling piece, with dowel-pins, draw and rapping
plates. If a system of draw-irons is not rigidly carried out,
moulders will not trouble themselves to take care of patterns.
In conclusion, I will say on the subject of patterns and castings,
that a learner must depend mainly upon what he can see and what is
explained to him in the pattern-shop and foundry. He need never
fear an uncivil answer to a proper question, applied at the right
time and place. Mechanics who have enough knowledge to give useful
information of their business, have invariably the courtesy and
good sense to impart such information to those who require it.
An apprentice should never ask questions about simple and obvious
matters, or about such things as he can easily learn by his own
efforts. The more difficult a question is, the more pleasure a
skilled man will take in answering it. In short, a learner should
carefully consider questions before asking them. A good plan is
to write them down, and when information is wanted about casting,
never go to a foundry to interrupt a manager or moulder at melting
time, nor in the morning, when no one wants to be annoyed with
questions.
I will, in connection with this subject of patterns and castings,
suggest a plan of learning especially applicable in such cases,
that of adopting a habit of imagining the manner of moulding, and
the kind of pattern used in producing each casting that comes under
notice. Such a habit becomes easy and natural in a short time, and
is a sure means of acquiring an extended knowledge of patterns and
moulding.
A pattern-maker no sooner sees a casting than he imagines the kind
of pattern employed in moulding it; a moulder will imagine the plan
of moulding and casting a piece; while an engineer will criticise
the arrangement, proportions, adaptation, and general design, and
if skilled, as he ought to be, will also detect at a glance any
useless expense in patterns or moulding.
(1.) Why cannot the regular working drawings of a machine be
employed to construct patterns by?--(2.) What should determine
the quality or durability of patterns?--(3.) How can the
arrangement of patterns affect certain parts of a casting?--(4.)
What means can be employed to avoid inherent strain in
castings?--(5.) Why is the top of a casting less sound than the
bottom or drag side?--(6.) What are cores employed for?--(7.)
What is meant by venting a mould?--(8.) Explain the difference
between green and dry sand mouldings.--(9.) Why is sand employed
for moulds?--(10.) What generally causes the disarrangement
of cores in casting?--(11.) Why are castings often sprung or
crooked?--(12.) What should determine the amount of draught given
to patterns?--(13.) What are the means generally adopted to avoid
cooling strains in castings?